The separation and purification of rhodium (Rh) from other precious metals is one of the most difficult and pressing areas in precious metal refining. This situation arises mainly because of the complex solution chemistry in chloride-containing aqueous solutions. The complexes formed by rhodium in these types of solutions are such that modern recovery processes such as solvent extractions (SX), which have been implemented for the recovery of other platinum group metals (PGMs), cannot be easily applied to the recovery of rhodium. Thus far, no industrially acceptable solvent extraction system has been developed for rhodium.
Rhodium is often used in combination with other PGMs in catalysts. In the life of a catalyst, the catalyst may lose some or all of its activity. A catalyst may deactivate through the accumulation of a layer of carbon deposits, or coke. Coke accumulation typically occurs throughout the catalyst pore systems and physically blocks access to active sites. Further, metal agglomeration may occur, which can severely reduce catalyst activity. Still further, poisons (e.g., lead, arsenic, sulfur) may permanently deactivate the catalyst. In many cases, deactivated catalysts are regenerated so that they recover at least part of their initial activity.
Cycles of deactivation and regeneration may occur for many years. The catalyst may be regenerated in situ or removed for ex situ regeneration. In one strategy, a fixed bed or slurry bed reactor unit and a regenerator unit are paired in tandem, for simultaneous operation. After the catalyst is regenerated ex situ it is commonly loaded back to the same or another unit. This procedure has the advantage of reducing down time of the reactor. Alternatively, a catalyst may continuously recirculate between a reactor and a regenerator. Cost savings over fresh catalyst vary widely, but using regenerated catalyst can save 50-80% of the new catalyst cost.
A catalyst that has been through cycles of use and regeneration may, with time, lose the ability to be regenerated to an adequate level of activity, becoming a spent catalyst. This loss of regenerability may be due to incompleteness of the regeneration. For example, in an oxidative regeneration of a coked catalyst, sulfur in the coke is typically not removed to as low a level as coke is removed during regeneration. Further, sulfates associated with alumina supports are typically not removed, nor are metal poisons. Permanent loss of acceptable activity may also occur through sintering or other structural changes.
Often a spent catalyst is discarded. However, a spent catalyst, if discarded, represents a loss of precious material, such as rhodium. Further, use of landfills for such disposal is problematic. For example, available landfills have decreased in number by 75% in the past 20 years, a trend that is expected to continue. Further, environmental liability can reach unacceptable levels if the landfill releases toxins to the environment. Still further, the environmental protection agency (EPA) “Land Ban” imposes restrictions on disposal.
Thus it is desirable to have a method for reclamation of catalyst materials. Reclamation is the process of recovering and recycling a material. For a PGM-containing catalyst, reclamation is particularly desirable for economic reasons. For example, a single drum of spent catalyst may contain thousands of dollars worth of valuable metals, such as rhodium, platinum, palladium, iridium, ruthenium, and osmium.
In particular, it is desirable to have a method for reclamation of rhodium from a spent catalyst. Rhodium is a relatively scarce material and is accordingly rather expensive. The costs of the entire catalytic process could be reduced appreciably by recovery of the rhodium from spent catalysts and subsequent recycling of the metal.
Because multiple PGMs are often used together, it is important to devise techniques to separate them and to purify and recover each of the metals separately. Originally, PGMs were separated after dissolution in oxidizing chlorine leach liquors by the application of a series of precipitation-dissolution steps adopted from analytical chemistry methods. This was the most common route until the middle nineteen seventies. Since then, the major refining companies have considerably modified their processes by implementing the more efficient separation technique of solvent extraction, and to a lesser degree, ion exchange.
In virtually all precious metal recovery systems, rhodium is the last metal recovered through a complicated precipitation technique rather than through the more modern and efficient technique of solvent extraction. The precipitation scheme-dissolution scheme for the recovery of rhodium is not considered satisfactory by most PGM refiners because of its numerous drawbacks. It is a lengthy process, sometimes taking as long as 4 to 6 months for the final recovery of pure rhodium metal and therefore, there is a high value of metal that is locked up in the processing plant. The technique is also quite tedious, as the precipitation must be carried out a number of times in order to ensure that the final product is of acceptable purity and this makes the overall process labor intensive and costly.
In the precipitation-purification method, the first step involves the formation of the nitrite complex [Rh(NO2)6]3− from RhCl63−. Because this complex is extremely stable to hydrolysis, the impure rhodium-containing solution can be subjected to neutralization with NaOH in order that some of the impurities be precipitated through hydrolysis. After a filtration stage, the rhodium in solution is precipitated with ammonium and sodium (from the NaOH) as Na(NH4)2[Rh(NO2)6], which is a partially selective precipitation step over the other PGMs that may also be present in the rhodium solution. For this precipitation, however, it is important that a high concentration of ammonia be used in order to suppress the solubility of this rhodium complex to achieve almost complete rhodium precipitation. After another filtration stage, the precipitate is redissolved in HCl and, depending on the purity of the solution, the process recommences with the nitrating step.
It is this cycle of precipitation-dissolution stages that renders this process inefficient and labor-intensive. Once the ammonia-nitrite rhodium complex is of acceptable purity, the final dissolution in HCl is followed by the precipitation of rhodium with ammonia to give (NH4)3[RhCl6]. It is not only important that the concentration of ammonia be high to suppress the solubility of the rhodium compound, but also that the chloride concentration be high, since it is the hexachloro-complex that is precipitated and therefore the hexachloro-complex must be available in solution. The last step involves the reduction of rhodium to the metallic state either directly from this solution with formic acid or by calcining the complex in the presence of H2 gas at about 1000° C.
As described above, rhodium metal is of high value, and with rapidly increasing demand for catalysts that utilize rhodium, the need to develop more efficient recovery processes such as solvent extraction for rhodium is becoming more urgent. Particularly, a method for recovering rhodium from solid spent catalysts is needed. The difficulty in developing such systems, however, lies in the chemical complexity of rhodium in Cl-containing aqueous solutions.
The main oxidation state of rhodium is +III, although +I and others are known to exist, though to a much lesser extent. The anionic complexes of rhodium are more labile than those of other PGMs, whereas the cationic and neutral complexes are quite inert.
Rhodium (III) readily forms octahedral complexes, as do most d6 configurations, with anions, halides, and oxygen-containing ligands. In terms of solvent extraction, highly charged RhCl63− ions are particularly difficult to extract due to steric effects because it is difficult to pack three organic molecules around a single anion.